Page 373 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 349

3.3.1
Drilling Problems [ ]

Undergauge hole due to squeezing shales. In shales that “relax” some
time after being drilled through, high overpulls may be seen when trip-
ping out. If a top drive is not being used, it is a time consuming and
risky operation to backream out and it may be possible to work out
carefully without having to make up the kelly. Start by overpulling no
more than half the BHA weight then slack off until free—even if this
means slacking off all of the string weight. If you can come free easily
going down, repeat with slightly more overpull (about 5-10 k more),
but ensuring each time that you can come free by slacking off. Note the
distance the string has to be lowered to come free; this will normally
be at least the amount to lower the string from the up-drag weight to
the down-drag weight. Do not pull up enough to trip the jar while
working the string; this is likely to pull the BHA into the undergauge
hole with enough force to completely stick the string.
Continue to work out of the hole. When the connection to break
for racking back is above the table, keep working up until the connec-
tion is so far above the table that if the hole becomes tight after break-
ing off the stand, the string can be lowered enough to come free. This
will be the distance noted above plus the normal tool joint height
above the table when breaking the connection.
With this type of problem, note the depths on the daily report and
take special care when passing those depths on the trip in. It often hap-
pens that once you have worked out through the tight spots, the prob-
lem will not be seen again.
If salt is squeezing in above, try programming wiper trips back
through the salt after a short time, say 15 to 20 hours after the previ-
ous trip or wiper trip. This may give some overpulls on the first wiper
trip through, but often these do not subsequently recur.
If the string becomes stuck in undergauge hole, work and jar the
string in the opposite direction to that when the string got stuck. Do
not jar the string both ways, this is a waste of time. If the string became
stuck pulling up, set down/jar down and, if possible, apply torque to
the string if setting down alone does not free the string. This can only
be done if the kelly bushing is already in the rotary table or if using a
top drive; do not attempt to use the slips, since there will be insuffi-
cient weight available to set down on them.
Often when working through these troublesome shales, it is better
not to circulate but to work out carefully without rotating, taking care
not to get stuck. Circulating sometimes seems to make it harder to

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